Time-of-flight mass spectrometry (TOFMS) is an analytical process that determines the mass-to-charge ratio (m/z) of an ion by measuring the time it takes a given ion to travel a fixed distance after being accelerated to a constant final velocity. There are two fundamental types of time-of-flight mass spectrometers: those that accelerate ions to a constant final momentum and those that accelerate ions to a constant final energy. Because of various fundamental performance parameters, constant energy TOF systems are preferred.
A previously known constant kinetic energy TOF mass spectrometer is shown in FIG. 1A. Ions are created in a region typically referred to as the ion source. Two ions with masses M1 and M2 have been created as shown in FIG. 1A. A uniform electrostatic field created by the potential difference between repeller lens 10 and ground aperture 11 accelerates ions M1 and M2 through a distance s. After acceleration, ions pass through ground aperture 11 and enter an ion drift region where they travel a distance x at a constant final velocity prior to striking ion detector 12.
The time-of-flight of the ions can be measured to calculate their mass-to-charge ratio values. For example, referring to FIG. 1A, within the ion optic assembly, accelerating electrical field (E) is taken to be the potential difference (V) between the two lens elements (10 and 11) as applied over acceleration distance s, (E=V/s). Equation (1) defines the final velocity (v) for ion M1 with charge z. The final velocity of ion M2 is determined in a similar manner.                     v        =                              (                                          2                ⁢                s                ⁢                                  xe2x80x83                                ⁢                E                ⁢                                  xe2x80x83                                ⁢                z                                            M                1                                      )                                1            /            2                                              (        1        )            
Inverting equation (1) and integrating with respect to distance s yields equation (2), which describes the time spent by ion M1 in the acceleration region (ts)                               t          s                =                                            (                                                M                  1                                                  2                  ⁢                  Esz                                            )                                      1              /              2                                ⁢                      (                          2              ⁢              s                        )                                              (        2        )            
The total time-of-flight for ion M1 (t1) is then derived by adding ts to the time spent during flight along distance x (the ion drift region). Time ts equals the product of the length of free flight distance x with 1/v, as shown in Equation (3).                               t          t                =                                            (                                                M                  1                                                  2                  ⁢                  Esz                                            )                                      1              2                                ⁢                                    (                                                2                  ⁢                  s                                +                x                            )                        2                                              (        3        )            
Rearranging equation (3) in terms of M1/z yields equation (4)                                           M            1                    z                =                              2            ⁢                          t              t              2                        ⁢            Es                                              (                                                2                  ⁢                  s                                +                x                            )                        2                                              (        4        )            
For all TOFMS systems, E, s, and x are intentionally held constant during analysis, thus equation (4) can be reduced to equation (5).                                           M            1                    z                =                  k          ⁢                      xe2x80x83                    ⁢                      t            t            2                                              (        5        )            
Equations (1)-(5) simplify the TOFMS process by assuming that all ions are created at the same time, within the same location, and have no initial velocity prior to acceleration. Routinely, this is not the case and in many instances, variations in formation time, original location, and initial velocity (also referred to as initial energy) are often demonstrated for various ions of a given m/z population. Such variation ultimately limits the mass resolving power of the instrument. Mass resolving power is typically defined as the ability to determine subtle differences in m/z.
For a TOFMS system, mass resolving power R is mathematically defined by equation (6), where dm and dt are the respective full mass or full temporal width of a measured signal at its half magnitude.                     R        =                              m            dm                    =                      T                          2              ⁢              d              ⁢                              xe2x80x83                            ⁢              t                                                          (        6        )            
Ultimately, factors that limit mass resolving power are dictated by the ionization means, geometry of the ionization source, geometry and stability of the TOF mass spectrometer, as well as the nature of the sample itself. Various strategies have been adapted to improve mass resolving power in time-of-flight mass spectrometry.
Another example of a TOF mass spectrometer is shown in FIG. 1B. The TOF mass spectrometer shown in FIG. 1B is an orthogonal extraction device. In the device, ions are generated from ion source 20 and directed to repeller lens 22 via RF ion guide 21. A uniform electrostatic field created between repeller lens 22, extractor lenses 29, and ground apertures 28 accelerate ions. After acceleration, ions pass through ground apertures 28 and enter an ion drift region along path 35 where they travel through reflectron 27. Reflectron 27 functions to narrow ion energy spread, and then it redirects the ions to detector 26.
The output signal of ion detector 26 can be an analog signal, which is then converted to a digital signal. The analog-to-digital conversion may be accomplished, for example, using a time-interval recording device, such as a time-to-digital converter (TDC). For instance, detector 26 outputs a signal to high speed time-to-digital converter (TDC) 24 when an ion impacts its detecting surface. TDC 24 converts analog signals from detector 26 to digital information suitable for software processing at stage 25. TDC 24 records a single impulse when the detector 26 output signal exceeds a predetermined threshold. HV pulser 23 indicates to TDC 24 the start of an ion detection cycle when the repeller lens 22 starts to accelerate the ions.
Previously known systems have employed means for providing gain in the output signal of detector 26 prior to digitization. Such gain has been provided by primary ion to secondary product or primary ion to secondary electron conversion prior to striking an electromissive detector surface. Primary ions are converted to secondary products through the mechanisms of surface induced dissociation, generating ion and neutral fragments, and/or fast ion bombardment of solid surfaces, creating sputtered products. Primary ions can also be converted to secondary electrons by directing them to strike a metal of low work potential, ultimately releasing low energy electrons. These secondary products are then directed to strike an electromissive device, creating an amplification cascade provided by the generation of secondary, tertiary, quaternary, etc. electrons.
The probability of producing an output signal from the detector 26 decreases with increasing time-of-flight (and also increasing m/z values). As shown in FIG. 2 as ion m/z increases, the ion-to-electron conversion probability decreases.
Ions are more likely to be detected by a detector if they have high velocities. Ions with high m/z values have greater mass and have lower velocities than ions with low m/z values. Consequently, ions with high m/z values have a lower probability of generating secondary charged particles such as electrons in the detector and have a lower probability of being detected by the detector than ions with low m/z values. For example, FIG. 2 depicts the ion to electron conversion probability for ions of various mass-to-charge ratio values (m/z) at two different kinetic energy levels: 50 KeV (line 30) and 25 KeV (line 31). As shown in FIG. 2, the ions with higher kinetic energy (line 30) are more likely to produce electrons than ions with low kinetic energy (line 31).
Also, ions are less likely to arrive at the detector if they remain in flight for longer periods of time. Ions with high m/z values have a higher mass and take a longer time to arrive at the detector than ions with low m/z values. Because ions with high m/z values remain in flight longer than ions with low m/z values, there is an increased chance that the ions may not arrive at the detector. Accordingly, the probability of transporting ions to the detector decreases as the m/z value of an ion increases. The decreased probability often results in shorter peaks in the mass spectrum signal at high m/z values than would be the case if all ions had the same chance of reaching the detector.
Furthermore, in TOF mass spectra, empirical data indicate that peaks tend to widen with increasing with time-of-flight values (and m/z values). A number of factors can contribute to increasing peak widths including differences in the initial velocity of the ions of a given m/z value, differences in the initial spatial distributions of the ions, slight differences in the chemical composition of the analytes, etc. As ions are in flight for longer periods of time, it is believed that factors such as initial velocity distributions can become more pronounced resulting in wider time-of-flight distributions in the mass spectrum signal. If left uncorrected, the resulting peaks in the mass spectrum signal are shorter and wider at the end of the mass spectrum signal than at the beginning of the mass spectrum signal, even though the areas of all peaks may indicate that substantially the same number of analyte ions were detected for each of the peaks.
In sum, the peaks in the mass spectrum can be short and wide at high m/z values, and tall and thin at low m/z values. This visual distribution of peak shapes can be problematic as one of the crucial steps in analyzing a mass spectrum signal is identifying peaks of potential analyte ions in the mass spectrum signal. The thinner, longer peaks at the beginning of the mass spectrum signal tend to dominate the visual presentation of the mass spectrum signal and the viewer""s eyes. The visual presentation gives the impression that the peaks at higher m/z values are not present even though the areas of those peaks would show that the ions forming those peaks were detected in substantially equal number as the ions forming the longer, thinner peaks at the beginning of the mass spectrum signal. It is possible that some peaks, and consequently some analytes at high m/z values may not be identified.
Even a xe2x80x9cpeak pickingxe2x80x9d algorithm may not be able to identify the shorter, wider peaks at the end of the mass spectrum signal. A xe2x80x9cpeak pickingxe2x80x9d algorithm can automatically identify peaks in a mass spectrum signal using predetermined criteria such as a minimum signal-to-noise ratio. The shorter, wider peaks can blend with noise thus making it difficult for a peak picking algorithm to find peaks of potential significance. Automated peak picking algorithms are desirable, but optimization of the algorithms, for example, to function well both for high intensity, narrow peaks at short time-of-flight values and low-intensity broad peaks at long time-of-flight values is difficult.
In view of these problems, it would be desirable to produce mass spectrum signal data with more clearly defined peaks, especially at high m/z values so that the peaks can be identified more easily by a user or an algorithm.
Embodiments of the invention address these and other problems.
Embodiments of the invention are directed to methods for processing a signal that is indicative of the mass-to-charge ratio values of ions from a detector. Other embodiments of the invention are directed to computer readable media and mass spectrometers.
One embodiment of the invention is directed to a method for digitally processing time-dependent signal data, the method comprising: (a) receiving the time-dependent signal data in memory, wherein the time-dependent signal data represent a time-dependent signal, and wherein the time-dependent signal data include representations of time-of-flight values of ions, or values derived from time-of-flight values of ions; and (b) scaling the time-dependent signal data with a time-dependent scaling function.
Another embodiment of the invention is directed to a computer readable medium comprising: (a) code for receiving time-dependent signal data in memory, wherein the time-dependent signal data represent a time-dependent signal, and wherein the time-dependent signal data include representations of time-of-flight values of ions, or values derived from time-of-flight values of ions; and (b) code for scaling the time-dependent signal data with a time-dependent scaling function.
Another embodiment of the invention is directed to a mass spectrometer system comprising: (a) an ionization source that generates ions; (b) a mass analyzer that receives the ions from the ionization source, and focuses and accelerates the ions using electrostatic fields toward an ion detector; (c) an ion detector with a detecting surface that detects the ions and produces a time-dependent signal; (d) a digital converter adapted to convert the time-dependent signal from the ion detector into time-dependent signal data; (e) a digital computer including a memory, the digital computer configured to process the time-dependent signal data according to the steps of (i) receiving the time-dependent signal data in the memory, wherein time-dependent signal includes representations of the time-of-flight values of the ions, or values derived from time-of-flight values of the ions, and (ii) scaling the time-dependent signal data with a time-dependent scaling function.
These and other embodiments of the invention are described in further detail below.